1. Field of the Invention
The present invention relates to flat display devices, such as organic electroluminescence display devices and inorganic electroluminescence display devices.
2. Description of the Related Art
Progress has been made in recent years in developing flat displays of the self-luminescence type such as organic electroluminescence displays (hereinafter referred to as “organic EL displays”) and inorganic electroluminescence displays (hereinafter referred to as “inorganic EL displays”). Use of organic EL displays, for example, in portable telephones is under study.
FIGS. 6 and 7 show an organic EL display, which is fabricated by forming an organic hole transport layer 15 and an organic electron transport layer 16 on opposite sides of an organic luminescent layer 14 to provide an organic layer 13 on a glass substrate 11 serving as a base, and forming anodes 12 and cathodes 17 on opposite sides of the organic layer 13. The organic luminescent layer 14 is caused to luminesce by applying a predetermined voltage across the anode 12 and the cathode 17.
The anodes 12 are made from transparent ITO (indium tin oxide), and the cathodes 17, for example, from an Al—Li alloy. The electrodes of each type are prepared in the form of stripes to intersect those of the other type in the form of a matrix. The anodes 12 are used as data electrodes, and the cathodes 17 as scanning electrodes. With one of horizontally extending scanning electrodes selected, voltage in accordance with input data is applied to data electrodes extending perpendicular to the scanning electrode, whereby the organic layer 13 is caused to luminesce at the intersections of the scanning electrode and the data electrodes to give a display of one line. The scanning electrodes are changed over one after another in the perpendicular direction to scan the matrix in the perpendicular direction to give a display of one frame.
The methods of driving such organic EL displays include the passive matrix driving method wherein the scanning electrodes and the data electrodes are used for time division driving, and the active matrix driving method wherein each pixel is held luminescent for one vertical scanning period.
On the other hand, FIGS. 10 and 11 show an inorganic EL display, which comprises a substrate 110 of glass or ceramic serving as a base, an inorganic layer 130 composed of an inorganic luminescent layer 140 and a dielectric layer 150 formed on one side of the layer 140, and first electrodes 120 and second electrodes 170 which are arranged respectively on opposite sides of the inorganic layer 130. AC voltage is applied across the first electrode 120 and the second electrode 170 to cause the inorganic luminescent layer 140 to luminesce.
It is known that like organic EL displays of the passive matrix driving type, the inorganic EL display is adapted for time division driving using scanning electrodes and data electrodes. While the organic EL display is driven with direct current for passing current through the luminescent layer thereof, the inorganic EL display is driven with alternating current for passing current through the luminescent layer thereof.
In the organic EL display of the active matrix driving type, each organic EL element 20 providing one pixel 10 has a first transistor TR1 performing an on/off function, a second transistor TR2 for converting input data to a current value, and a capacitance element C performing a memory function as shown in FIG. 8. A drive line 4 is connected to the source of the second transistor TR2. A drive circuit 6 comprises a gate driver 61 for driving the scanning electrodes, and a source driver 62 for driving the data electrodes.
First, the gate driver 61 successively applies voltage to the scanning electrodes to bring the first transistors TR1 connected to the same scanning electrode into conduction. Data (input signal) is fed to the source driver 62 as timed with this scanning. Since the first transistor TR1 is in a conducting state at this time, the data is stored in the capacitance element C.
The operating state of the second transistor TR2 is dependent on the charge of data stored in the capacitance element C. For example, suppose the second transistor TR2 is brought into an operative state. Current is then supplied to the organic EL element 20 via the second transistor TR2. As a result, the EL element 20 luminesces. This luminescent state is maintained over one vertical scanning period.
The organic EL display 1 is self-luminescent, as stated above, and the required pixels only need to be turned on, so that the display is adapted to reduce power consumption unlike the liquid crystal display wherein the backlight needs to be on at all times. This is also true with all flat displays of the self-luminescent type including inorganic EL displays.
For example, when white is displayed in a central area B of the screen of the organic EL display 1 shown in FIG. 9, with a given intermediate color presented in the area surrounding this area B, the upper and lower areas A and A′ of drive lines (power supply lines) indicated in broken lines have luminance lower than the luminance corresponding to the input signal, unlike the left and right areas showing the intermediate color, hence the problem of so-called crosstalk. Although the central area B has luminance lower than the luminance corresponding to the input signal, the lower luminance is not conspicuous because there is nothing to be compared with the area.
Accordingly, an object of the present invention is to provide a flat display device in which crosstalk is suppressed.